Well, Negan's end in the comics isn't straightforwardly due to one specific killing act. It's more complicated and involves multiple factors and characters' actions.
Well, Negan first appears in the series as a major antagonist. He has a very distinct personality. He's confident, often to the point of being cocky. His group, the Saviors, would extort other communities for resources. He uses fear as a tool to control those around him. His actions have a huge impact on the other characters in the show, like Rick and his group.
Rick is a very likely character to interact with Negan in hurt Negan fanfiction. Their relationship has been a central part of the original story, and in fanfiction, their interactions could be further explored, especially if Negan is in a 'hurt' state. Another character could be Carl. Carl had his own complex relationship with Negan, and in fanfiction, their dynamic could be developed in new ways.